riscv : add riscv qemu virt support and fix fs bit error in mstatus

ports/risc-v64/gnu/example_build/qemu_virt/board.c

@@ -0,0 +1,32 @@
+#include "plic.h"
+#include "hwtimer.h"
+#include "uart.h"
+#include <stdint.h>
+#include <stddef.h>
+
+void *memset(const void *des, int c,size_t n)
+{
+    if((des == NULL) || n <=0)
+            return (void*)des;
+    char* t = (char*)des;
+    int i;
+    for(i=0;i<n;i++)
+        t[i]=c;
+    return t; 
+}
+
+
+int board_init(void)
+{
+	int ret;
+	ret = plic_init();
+	if(ret)
+		return ret;
+	ret = uart_init();
+	if(ret)
+		return ret;
+	ret = hwtimer_init();
+	if(ret)
+		return ret;
+	return 0;
+}